A discrete lattice plane analysis of the interfacial energy of coherent F.C.C. H.C.P. interfaces and its application to the nucleation of γ in AlAg alloys

The discrete lattice plane (DLP) model is used to calculate the chemical interfacial energy (γ) of coherent f.c.c.:h.c.p. interfaces of arbitrary orientation in AlAg alloys. The compositional diffuseness of the interface is neglected. The results of these calculations are used to develop polar γ-plots of an h.c.p. crystal within an f.c.c. matrix, obtained as a function of alloy composition. The Wulff construction on the γ-plot yield the equilibrium shapes taken to correspond to that of homogeneously formed h.c.p. γ′ critical nuclei in an f.c.c. α AlAg matrix. These shapes are then used to calculate the steady state homogeneous nucleation rate of γ′ as a function of alloy composition and reaction temperature. Although these calculations indicated that homogeneous nucleation of coherent γ′ should be feasible in the temperature region between the GP zone solvus and the metastable equilibrium (α/(α+γ′)) solvus, a concurrent TEM investigation showed that the only γ′ present was nucleated on dislocations. While competition with such heterogeneous nucleation may have inhibited homogeneous nucleation, it is probable that the nearest neighbor version of the discrete lattice plane model used significantly underestimated the energy of coherent α:γ′ boundaries in AlAg alloys.